Super-conductive magnet systems are used in many of the presently available and proposed Magnetic Resonance Imaging (MRI) diagnostic devices. Typically these magnets consist of a super-conducting coil immersed in a bath of liquid helium to maintain a low temperature of 4.2 degrees K. One of the problems with liquid helium cooled systems is that low temperature is maintained by the "Boil-Off" of the liquid helium that occurs when it's temperature reaches 4.2 degrees K. Due to the "Boil-Off", the helium has to be replaced at frequent intervals. Thus costly and bothersome replacement of the helium is required after relatively short time periods.
To reduce the effects of heat at room temperature on the helium liquid the bath is enclosed by a vacuum chamber. To further reduce the heat typically one or more heat shields (usually two) are placed in the vacuum space such that they completely enclose the bath of liquid helium, and the bath, the heat shields and the vacuum chamber are kept concentric, but, without touching each other by various supports having low thermal conductivity. In the prior art, two heat shields were used, and they were cooled to further reduce liquid helium loss using low temperature refrigerator systems, operating on a Gifford McMahon cycle, Stirling cycle or other thermodynamic cycle. The steady state operating temperature of these shields was governed by the heat balance between the cooling power of the refregirator device and the heat entering the shields by way of conduction and radiation.
In a well-designed MRI magnet a typical low temperature refrigerator can provide more cooling power than is required to reduce helium boil-off. This is because once the shield closest to the helium can is cooled below 20 degrees K, its further cooling is unnecessary. It is enough to make sure that the temperature of the shields does not rise above 20 degrees K. Cooling the temperature of the shields to 20 degrees K rather than to 10 degrees K surprisingly actually adds to the useful lifetime of the refrigerator. When the refrigerator is working at a low number of cycles per second with low cooling, the refrigerator's moving parts wear out at a slower rate; thus, the service interval of the refrigerator can be extended.
However there is a problem associated with radiation shields which is the appearance of eddy currents in the shields. These currents are set up by pulsed gradient magnetic fields during the image acquisition process. The gradient field is normally varied to compensate for these eddy currents and the magnetic field they create. However as the working parts of the refrigerator wear with time the refrigerator's cooling power decreases, and the shield temperature rises. As the temperature of the shield changes, it's electrical resistance changes; thus the size of the eddy current changes. This results in incorrect compensation, which causes a decrease in image quality. Therefore the refrigerator device will require servicing or the eddy current compensation will require recalibrating.
In both cases it will be necessary to put the MR system out of service and to use the time of a qualified person, thus causing a waste of money and time. In the prior art the use of an electric heater was suggested to maintain the constant temperature of the shields. The electric heater was attached to the shields. An example of such an apparatus can be found in EP-416-959-A. This solution while it does help to keep the temperature of the shields constant, has a number of disadvantages. One disadvantage of such a system is that it uses more cooling power from the refrigerator than necessary for cooling the radiation shields themselves; thus the parts of the refrigerator wear out faster, and need to be serviced sooner than if only the necessary cooling power was used. Another disadvantage of the above cited prior art is the need to add an electrical component (electric heater) to the shields themselves. This can adversely effect the homogeneity of the magnetic field of the MR system.
This invention is concerned with solving the above problems and others that are associated with the use of radiation shields in MR systems.